Niels Linnemann – författare

The programme of 'constructive axiomatics', promulgated by Hans Reichenbach in 1924, seeks to build up the architecture of our best theories of physics from basic axioms supposedly imbued with immediate and indubitable empirical content. Taking inspiration from Reichenbach, Hermann Weyl proposed his own 'causal-inertial' approach to the constructive axiomatization of Einstein's general relativity, according to which a relativistic spacetime can be constructed solely from the trajectories of light rays and freely-falling particles; this project, however, came to fruition only in 1972, with the constructive axiomatization of general relativity due to Ehlers, Pirani, and Schild ('EPS').One century since Reichenbach, and fifty years since EPS, Constructive Axiomatics for Spacetime Physics is a celebration of the constructive axiomatic methodology. It achieves four main tasks. First, it provides a thoroughgoing presentation of the EPS axiomatization, closing missing loopholes, identifying problematic axioms, and so forth—in this way, one gains a much-improved appreciation of the extent to which a causal-inertial approach to general relativity might succeed, and of what such an approach might offer. Second, it synthesizes and assesses the vast but disparate literature on constructive axiomatics which has arisen over the past century and sets the methodology in its proper philosophical context. Third, it generalizes the approach to apply to quantum spacetimes. And fourth, it applies the approach to the context of non-relativistic spacetime physics. All in all, the book demonstrates that constructive axiomatics is live-and-kicking; the book will become the go-to resource for this way of philosophizing about the nature of space and time.

The characteristic - Planck - energy scale of quantum gravity makes experimental access to the relevant physics apparently impossible. Nevertheless, low energy experiments linking gravity and the quantum have been undertaken: the Page and Geilker quantum Cavendish experiment, and the Colella-Overhauser-Werner neutron interferometry experiment, for instance. However, neither probes states in which gravity remains in a coherent quantum superposition, unlike - it is claimed - recent proposals. In essence, if two initially unentangled subsystems interacting solely via gravity become entangled, then theorems of quantum mechanics show that gravity cannot be a classical subsystem. There are formidable challenges to such an experiment, but remarkably, tabletop technology into the gravity of very small bodies has advanced to the point that such an experiment might be feasible in the near future. This Element explains the proposal and what it aims to show, highlighting the important ways in which its interpretation is theory-laden.